1. Field
The present invention relates generally to varactors, and more specifically to a programmable varactor device including a single varactor pair.
2. Background
As will be appreciated by a person having ordinary skill in the art, a varactor is generally a device designed to take advantage of variations in its reactance. A varactor may be thought of as a variable voltage capacitor. As a voltage is applied to a varactor, the capacitance of the varactor generally increases. Varactors may be used in various types of tuning circuits. As an example, varactors may be useful in certain oscillator circuits, such as oscillator circuits commonly used in communications devices. The operating frequency of an LC oscillator circuit, for example, may be controlled or tuned by varying the voltage across the terminals of a varactor. As another example, a varactor may be used in the tuning mechanism of a radio receiver or another device requiring frequency tuning.
Voltage controlled oscillator (VCO) circuits are well known in the art and are utilized in a number of applications. For example, VCO circuits are used in phase-locked loop (PLL) circuits in high frequency applications such as wireless communications. A PLL is a component used in communications circuitry that enables communications equipment to quickly “lock” onto a specifically selected frequency, typically the carrier frequency over which communications are sent. This fast locking ability is particularly important for devices such as cellular telephones, where the cell phones are desired to instantly switch carrier frequencies when traveling through different cellular zones or “cells”. A VCO is an essential component of a PLL, whose output voltage is controllable by the application of an input control voltage. However, a VCO is very sensitive to fluctuations in a control voltage. The sensitivity of a VCO is typically expressed as MHz per volt.
Typically, a VCO circuit includes a variable element such as a capacitor that may be varied to adjust the frequency of an output signal of the VCO circuit. In a LC tank based VCO circuit, the frequency of the VCO circuit is determined by the inductance (L) and capacitance (C) of the tank circuit. By utilizing a varactor to function as a capacitor in the LC tank circuit, the capacitance of the VCO circuit can be varied by changing the voltage applied to the varactor. Thus, the frequency of the LC tank based VCO circuit is varied accordingly.
FIG. 1 illustrates a conventional distributed bias varactor device 100 including a plurality of varactor pairs Cvar1/Cvar1′, Cvar2/Cvar2′ . . . Cvarn/Cvarn′. As illustrated in FIG. 1, each varactor pair Cvar1/Cvar1′, Cvar2/Cvar2′ . . . Cvarn/Cvarn′ is coupled to a tuning voltage Vtune, which may be received from a PLL. Furthermore, each varactor pair Cvar1/Cvar1′, Cvar2/Cvar2′ . . . Cvarn/Cvarn′ is coupled to a bias voltage through an associated resistor (e.g., varactor Cvar1 is coupled to bias voltage Vbias1 through resistor Rb1). As will be understood by a person having ordinary skill in the art, each resistor within varactor system 100 may increase noise and, as a result, for each varactor within varactor system 100, the noise is increased. Moreover, because each varactor is capacitively coupled to a tank voltage (e.g., Cvar1 is coupled to tank voltage Vtank+ via fixed coupling capacitor Cc1), the tuning range of varactor system 100 may be reduced.
A need exists for a programmable varactor, which may provide increased tuning range and decreased noise compared to a conventional varactor system.